A scanning probe microscope analyzes a succession of small areas which make up the test sample, in contrast to a conventional optical microscope, which forms an image of the sample simultaneously over the entire field of view of the sample. In a scanning probe microscope, the information pertaining to each small area is stored and processed, and recombined into a composite image, much as a television reconstructs an image using a "flying" spot on a picture tube. Scanning probe microscopes figure prominently in a number of areas of research and technology where interactions and features of very small size are important. FIG. 1 shows a schematic diagram of a scanning probe microscope 10 which includes a probe 12 for analyzing a sample 15. The measured data is processed and recombined into a composite image on screen 16.
An important type of scanning probe microscope uses a high precision mechanical actuator to scan the active region of the microscope over the sample, or conversely, to scan the sample past the active region. The microscope's active region may, for example, be defined by a focused beam of light or other energy, or it may be defined by a very local interaction between a probe tip and the sample. Two examples of the latter type of microscope are the Scanning Tunneling Microscope (STM) and the Atomic Force Microscope (AFM). Using short range interactions between the probe and the sample, and by carefully controlling the relative motion between the probe and the sample, it is possible to extend well beyond the resolution limits of conventional optical microscopes, down even to atomic dimensions.
FIG. 2 is a schematic drawing of a typical scanning probe (e.g. STM or AFM) microscope actuator and probe tip. To achieve the highest possible sensitivity and resolution while achieving the least possible distortion of the geometrical features of the object, the scanning motion of the sample or probe must be very well controlled.
Piezoelectric transducers of a variety of compositions and geometries have been used for precise motion control. (The abbreviation "PZT" is used in the drawings to designate piezoelectric transducers in a general sense, although technically the abbreviation "PZT" refers specifically to piezoelectric transducer ceramic material composed of lead, zirconium, and titanium.) Such transducers are capable of controlled motion with resolutions of 0.1 nanometer or better, with ranges of motion of several micrometers. However, there are several problems associated with the use of piezoelectric transducers, including hysteresis, non-linearity (sensitivity which varies with range), changes in sensitivity over time, drift or creep (position which changes over time), and mechanical (spring-mass) resonances of the scanning element at relatively low frequencies. For example, over large scan ranges (e.g., over 10 micrometers), hysteresis and non-linearity may cause severe distortion of the scanned image. Furthermore, spring mass resonances may limit scanning speeds, as such resonances must be allowed to decay before scanning is continued. Prior scanning probe microscopes have either accepted errors introduced by nonlinearity and/or hysteresis in actuators or have partially corrected errors due to non-linearity by means of error-correcting algorithms in a digital computer. (It is very difficult to effectively eliminate errors due to hysteresis in this manner).
Prior techniques employed to overcome such problems have not been entirely successful, especially where it has been desirable to extend the scanning range of the device beyond a few micrometers. Some researchers have corrected scanned images with reference to interferometric or capacitance based distance measuring techniques. However, those techniques tend to be cumbersome, sensitive to alignment, and expensive. For example, see "A Scanning Tunneling Microscope with a Capacitance-based Position Monitor", by Griffith et al., Journal of Vacuum Science Technology, pages 2023-2027 Nov./Dec. 1990. This reference describes removal of inaccuracies in probe position measurements. The described device is very sensitive to tilt produced by the piezoelectric transducer. Use of capacitive sensors to achieve two-dimensional sensing as described therein is cumbersome because it is difficult to provide all of the necessary mechanical components in a suitably small volume. Furthermore, capacitive position sensors, because they sense very small changes in the ambient electrical field, are vulnerable to electromagnetic interference, which is very undesirable because the capacitive position sensor must operate in a very noisy electrical environment.
As an example, the problem of drift is particularly acute when the sample to be imaged is small and complicated, such as are most biological molecules, for example, proteins, deoxyribonucleic acid ("DNA") or the like. A molecule of DNA on the average is about 3 nanometers wide and about 100 nanometers long. At a typical STM scanning speed of three lines/second, an image of the DNA molecule can be generated in 1-2 minutes. Unfortunately, the piezoelectric transducer drift is commonly in the range of 5-10 nanometers per minute, which severely affects the resolution of the DNA molecule image. The piezoelectric transducer drift can be reduced to about 1 nanometer per minute if a large scan (on the order of 100 nanometers.times.100 nanometers) is used. However, that size of scan is too large for biological molecules, which are usually scanned in a field of about 10 nanometers.times.10 nanometers. With that size of scan and a piezoelectric transducer drift of 5-10 nanometers per minute, the image of a DNA molecule would be a blur.
Thus, the prior art has suffered from a lack of a suitable position sensor for measuring the position of a scanned element. It is important that such means have the correct combination of range, stability, resolution, physical compactness, low mass, simplicity, high frequency bandwidth, relative insensitivity to tilt and misalignment, low cost, and freedom from creep (i.e., drift), nonlinearity, and hysteresis. Availability of such a position sensing system would greatly improve scanning microscopy by correcting the defects of piezoelectric transducer-like actuators, either through after-the-fact corrections to measured data or through real-time servomechanical control of the moving element.